In a (colour) display tube the earth's magnetic field deflects the electron paths, which without any measures may cause mislanding of the electrons on a phosphor and discolouration of the picture. Particularly the component of the earth's magnetic field in the direction of the axis of the display tube (commonly referred to as the axial field) plays an important role in this respect, which may become manifest as a lack of colour or even as colour impurity in the comers of the display screen.
A known measure of reducing mislandings due to the earth's magnetic field is the use of an internal magnetic shield. The shape of such a shield roughly follows the contours of the envelope of the display tube. This means that the (funnel-shaped) shield has two long trapezoidal sides which are parallel to the long axis (the x axis) of the display screen and two short trapezoidal sides which are parallel to the short axis (the y axis) of the display screen.
The short sides of the shield often have triangular recesses at the gun side so as to reduce mislanding in the corners due to the axial field. When relatively small tubes and a relatively large pitch of the pixels of the phosphor pattern on the display screen are used, an acceptable result is achieved in this way. When pixels are used on a display screen with a phosphor pattern consisting of (for example, hexagonally arranged) phosphor dots, notably in the case of larger display tubes and/or a smaller pitch of the phosphor pixels, this type of solution does not, however, guarantee a sufficient colour purity.
The invention is based on the recognition that the mislandings in the y direction are larger than those in the x direction because the display screen has a width which is larger than its height. In display tubes in which the phosphors are arranged in accordance with a pattern of vertical rows, the y mislandings are not important. In high-resolution tubes the phosphors are, however, arranged in accordance with a (hexagonal) pattern of dots. Mislandings in the y direction are then as troublesome as those in the x direction. Since by nature the y mislandings are larger due to the aspect ratio of the screen, extra attention is to be paid to this in such tubes. This applies to an even greater extent to tubes having display screens with a 9:16 aspect ratio, which are more elongate than the conventional display screens having a 3:4 aspect ratio.